Yes, so coin holders will get some "free" BTG, but it's not necessary a good thing!

It's very likely that such a coin will be worthless anyway, and the btc-hodlers shouldn't need to bother about it. In such a sense, the news about a split is a neutral event; it could still be sort of a negative event as some btc-hodlers will waste time and energy to figure out if they own any BTG, if the BTG is valuable, how to extract and sell the BTG, etc.

At the other hand, if the coin does become valuable - the coin value has to come from somewhere, you can't sell the coin unless someone is willing to buy it. Those people buying it, do they buy it using bitcoins? If there is no extra flow of fiat into the ecosystem to buy BTG, the total market cap of BTC+BTG will be constant - if BTG gains value, BTC will drop (or grow less than what it otherwise would have grown).

Now why is BTC so valuable compared to altcoins? The network effect is the very most important thing, and if people "defect" from bitcoin and start using BTG instead, it will be bad for bitcoin.

So far, Bitcoin Cash has proved to be "free money" for BTC holders. BTC grew sharply after the split, and Bitcoin Cash has also been holding a strong value. I believe there has been a very strong covert marketing campaign behind Bitcoin Cash, including the inflow of fiat to pump the value artificially high. At one hand, Bitcoin Cash is sort of a threat to Bitcoin, but at the other hand I also think the split has been a bit of a relief - many bigblockers feel a renewed optimism now with Bitcoin Cash being "Bitcoin as it should be", and many smallblockers feel relieved to "get rid" of the most tocix hardblockers. With optimism, more fiat investments ...